If you want to build it off an existing helmet, your best bet is to use a BMX motorcross helmet. That's all that helmet really is, with some bulk added to it, and stuff like the sun-visor notched in the middle.
However, it seems to me that it's be better to just make the helmet the same way you're going to make the actual armor itself.. It's just armor for your head.
Plus it's like you might need an undersized BMX helmet and then scoop out all the padding and stuff. When they determined the size for the masterchief model, they just kinda' threw a 3d helmet up there with no concern for actually having a head inside of it. It's unrealistic, but then again it was for a video game, not an actual person.
One note about doing any of this stuff though is that the programs they use to display the masterchief are different between cutsceens and the gameplay itself. They haven't mastered a way to make the perspective angles work consistently either, which basically means that from SOME angles, at certain distances, a helmet might look right, and from other angles it might not.
Kinda' like the action figure's helmet. That's definately based on a BMX helmet, and from some angles is a perfect representation of what MC's helmet looked like when he was close to the "camera" from certain angles. If you see him from a slight distance, it changes the perspective angles, and makes the toy version look inaccurate.
For the record though.. the toy is NOT accurate, and is "wrong" in many ways. If you match it, you will feel disappointed whenever you play halo 2 multiplayer, because you'll see all the stuff you wanna do differently on your suit.